The effect of immobilization by plaster cast was studied on the activities of phosphoglucomutase (PGM), pyruvate kinase ( PyK ) and cAMP-dependent protein kinase in fast (extensor digitorum longus [EDL]), and slow (soleus) muscles of rats. In control untreated animals, PGM and PyK activities are 3 and 5 fold higher respectively in EDL than in soleus in correlation with the high glycolytic activity of fast muscles. During the four weeks period of immobilization a 20% decrease occurred in PGM activity, to which no limiting role in glycolysis is attributed, while PyK which has a regulatory function, showed a 35% decrease in EDL; at the same time in the soleus the activity of these enzymes did not change. The decrease of PGM and PyK activity in EDL diminished the difference between the slow and fast muscles, and it was evaluated as a tendency to dedifferentiation (transformation). The activity of cAMP-dependent protein kinase, in contrast to the glycolytic enzymes, was higher in the soleus than in the EDL and during immobilization it did not change significantly in either the muscles.